1. Field of the Invention
This invention relates to polyester multifilament yarn with high modulus and low shrinkage particularly useful for the textile reinforcement of tires. The yarn of the invention provides treated cords with the combination of exceptional dimensional stability and high ultimate elongation. A process for production of the multifilament polyester yarn is an aspect of the invention.
2. Description of the Prior Art
Polyethylene terephthalate filaments of high strength are well known in the art and are commonly utilized in industrial applications including tire cord for rubber reinforcement, conveyor belts, seat belts, V-belts and hosing.
Continued improvement in dimensional stability of polyester industrial yarns is desired to reduce tire sidewall indentations with current polyester monoply radial tires and to replace rayon in the bodies of radial tires and other reinforced industrial products. For the latter, the objective is to have strength and modulus equivalent to rayon at elevated tire service temperatures, while using up to 30 percent less material. While the current polyester cords have sufficient strength, their elevated temperature modulus is lower than desired for this application. U.S. Pat. No. 4,101,525 to Davis et al. provides a high strength multifilament polyester yarn with low shrinkage and work loss characteristics. Although Davis et al. does not provide treated cord data, yarns exhibiting the features taught by Davis do not meet the modulus requirements. Additionally, rapid cooling of the filament immediately after emerging from the spinneret can result in excessive filament breakage and thus yield yarn with poor mechanical quality. U.S. Pat. No. 4,491,657 to Saito et al. discloses high modulus, low shrinkage polyester yarn, but requires a low terminal modulus to achieve good yarn to treated cord conversion efficiency for such dimensionally stable yarns. The low terminal modulus is translated into the treated cord and results in a lower tenacity than the high terminal modulus cords of the present invention. Also, the process of Saito et al. requires high spinning speeds, which makes it difficult to incorporate this invention into a continuous spin-draw process.
U.S. Pat. No. 4,690,866 to Kumakowa et al. describes a means of making yarns which yield highly dimensionally stable treated cords using ultra high viscosity polymer. On a comparative experimental basis, i.e. utilizing our solvent system, the Kumakowa intrinsic viscosity (IV) values would be 5% higher than indicated in their patent, i.e. they required a minimum of 0.95 IV polymer by our measurements. Also, these cords have low terminal modulus and hence do not achieve the full tenacity benefit of a given polymer viscosity.